The HIG is written in reStructuredText, a lightweight markup language.
For example the chapter heading together with the first paragraph looks like this in reStructuredText:

Contribute
==========
The HIG is written in `reStructuredText <http://www.sphinx-doc.org/en/stable/rest.html>`_, a light weight markup language.
For example the chapter heading together with the first paragraph look in reStructuredText like this

The restructuredText of the full HIG is organized into several source files. You can view and modify these source files with any text editor.

The source files are hosted in a Git repo. Sphinx is used to generate HTML pages from these source files. Tasks and changes are organized via Phabricator.

Note

On every page of the HIG, there is a View page source link in the top right corner.